There are a number of special things you can inscribe (with the { command) on your equipment and inventory to make playing Angband more convenient.
n is a number from 0-9
c is a command from the underlying command set. (Almost, but not completely, like the orginial command set, which uses some keymaps so you don't need to know that ; is the walk command, and such. See the help file command.txt for these.)
!c - Ask for confirmation when you select this item to use with command c.
!* - Ask for confirmation when you select this item to use with any command.
@n - When you type n at a what object? prompt for any command, use this item. (If more than one item has this inscription, it uses the first one.)
@cn - As above, but only when selecting objects for command c.
For both of these, if the first matching inscription in your inventory is an illegal choice, it will stop there, not go on to a legal one.
These two are especially useful in macros, since they allow you to make a macro to use rod 2, for instance, and not have to worry about what inventory slot it is currently in. Or, for making macros of spells, it causes the macro to fail if all your copies of the appropriate book are destroyed, moving a different book into the same slot.
^c - While this item is equipped, ask for confirmation of command c.
=g - Pick this item up when you step on it.